(1) Field of the Invention
The present invention relates to a method and to an aircraft for minimizing the risks of the aircraft toppling while on the ground. The invention relates in particular to a rotary wing aircraft having a main rotor and a yaw movement control rotor, together with wheeled landing gear.
(2) Description of Related Art
A rotary wing aircraft has an airframe. A rotary wing aircraft is also provided with landing gear on which the airframe rests when the aircraft is standing on the ground. Such landing gear may be provided with a plurality of undercarriages, each having at least one wheel. The landing gear is then referred to as “wheeled landing gear”.
Furthermore, in a rotary wing aircraft, the airframe may carry a main lift rotor, and a yaw movement control rotor. For a helicopter type aircraft, the main rotor contributes to providing the aircraft both with lift and with propulsion.
The yaw movement control rotor is conventionally carried by a tail boom connected to the airframe. The yaw movement control rotor is thus located at the rear end of the aircraft. The yaw movement control rotor is commonly referred to as the “tail rotor”.
This yaw movement control rotor exerts lateral thrust seeking in particular to counter the yaw movement torque generated on the airframe by the main rotor.
Furthermore, the pilot can control this lateral thrust with the help of yaw movement control means for piloting yaw movements of the aircraft. Such yaw movement control means commonly comprise a pair of pedals acting on the collective pitch of the blades of the yaw movement control rotor.
The main rotor rotates substantially in a plane that can be tilted relative to the airframe. The pilot then controls the pitch of the blades of the main rotor cyclically in order to tilt the main rotor so as to direct the aircraft. For this purpose, the pilot has cyclic pitch control means, e.g. such as a cyclic stick.
The cyclic pitch control means may be subdivided into means for controlling lateral cyclic pitch enabling the main rotor to be tilted laterally, and means for controlling longitudinal cyclic pitch enabling the main rotor to be tilted longitudinally.
The aircraft also has collective pitch control means for controlling the pitch of the blades of the main rotor collectively, in particular to enable the pilot to cause the aircraft to move in elevation. For this purpose, the aircraft may include a collective stick.
On aircraft that are of sufficient size, the yaw movement control rotor is often high above the ground. This characteristic makes it possible to maximize the distance between the ground and the low point through which the blades of the yaw movement control rotor pass. This serves to minimize any risk of interfering with people on the ground.
With such a high position, any variation in the thrust from the yaw movement control rotor gives rise to a lateral toppling torque. On the ground, this toppling torque is compensated by opposing torque from the landing gear. The ground reaction on the undercarriages present on the side where the toppling torque applies increases, while the ground reaction decreases on the undercarriages present on the opposite side.
If the toppling torque is sufficient for the ground reaction on an undercarriage to become zero, then the undercarriage leaves the ground. The aircraft then falls over on the opposite side. Measuring the asymmetry of loading on the undercarriages thus gives an indication of proximity to the toppling limit.
When the pilot requests a large change in the thrust from the yaw movement control rotor, the induced toppling torque may be compensated by a roll moment. The roll moment may be produced by the main lift rotor tilting laterally in the opposite direction to the variation in the thrust from the yaw movement control rotor.
Such variations in the thrust from the yaw movement control rotor are used by the pilot to steer the aircraft along a path on the ground. Pilots are therefore taught during pilot training to co-ordinate control of the thrust from the yaw movement control rotor and control of the lateral tilting of the main rotor so as to avoid any risk of the aircraft toppling.
The problem becomes more complicated in the presence of wind. The thrust from the yaw movement control rotor does not depend only on its collective pitch, but depends also on the wind, and mainly on the lateral component of the wind speed. Fluctuations in the wind, whether in force or in direction, can then lead to a change in the toppling torque exerted on the aircraft while it is on the ground.
If the safety margin relative to actual toppling of the aircraft is not sufficient, and if the pilot does not react promptly, a gust of wind can lead to difficulty.
Furthermore, unlike variations in the thrust from the yaw movement control rotor under the control of the pilot, variations in said thrust caused by the wind can often be identified only by the effects they produce. For example, a slow change in wind direction can be very difficult to perceive, since it gives rise to a change in the equilibrium of the aircraft that takes place slowly.
Documents FR 2 787 763, FR 2 809 082, and DE 20 2007 010854 do not belong to the field of the invention and they are remote from the invention.
For example, document FR 2 787 763 relates to a device for recentering a flight control member.